| Network Working Group | J. Uberti |
| Internet-Draft | |
| Intended status: Standards Track | October 31, 2016 |
| Expires: May 4, 2017 |
WebRTC Forward Error Correction Requirements
draft-ietf-rtcweb-fec-04
This document provides information and requirements for how Forward Error Correction (FEC) should be used by WebRTC applications.
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In situations where packet loss is high, or perfect media quality is essential, Forward Error Correction (FEC) can be used to proactively recover from packet losses. This specification provides guidance on which FEC mechanisms to use, and how to use them, for WebRTC client implementations.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
By its name, FEC describes the sending of redundant information in an outgoing packet stream so that information can still be recovered even in the face of packet loss. There are multiple ways in which this can be accomplished; this section enumerates the various mechanisms and describes their tradeoffs.
This approach, as described in [RFC5956], Section 4.3, sends FEC packets as an independent SSRC-multiplexed stream, with its own SSRC and payload type. While by far the most flexible, each FEC packet will have its own IP+UDP+RTP+FEC header, leading to additional overhead of the FEC stream.
This approach, as descibed in [RFC2198], allows for redundant data to be piggybacked on an existing primary encoding, all in a single packet. This redundant data may be an exact copy of a previous packet, or for codecs that support variable-bitrate encodings, possibly a smaller, lower-quality representation. In certain cases, the redundant data could include multiple prior packets.
Since there is only a single set of packet headers, this approach allows for a very efficient representation of primary + redundant data. However, this savings is only realized when the data all fits into a single packet (i.e. the size is less than a MTU). As a result, this approach is generally not useful for video content.
Some audio codecs, notably Opus [RFC6716] and AMR [RFC4867] support their own in-band FEC mechanism, where redundant data is included in the codec payload.
For Opus, packets deemed as important are re-encoded at a lower bitrate and added to the subsequent packet, allowing partial recovery of a lost packet. This scheme is fairly efficient; experiments performed indicate that when Opus FEC is used, the overhead imposed is about 20-30%, depending on the amount of protection needed. Note that this mechanism can only carry redundancy information for the immediately preceding packet; as such the decoder cannot fully recover multiple consecutive lost packets, which can be a problem on wireless networks. See [RFC6716], Section 2.1.7 for complete details.
For AMR/AMR-WB, packets can contain copies or lower-quality encodings of multiple prior audio frames. This mechanism is similar to the [RFC2198] mechanism described above, but as it adds no additional framing, it can be slightly more efficient. See [RFC4867], Section 3.7.1 for details on this mechanism.
The following section provides guidance on how to best use FEC for transmitting audio data. As indicated in Section 8 below, FEC should only be activated if network conditions warrant it, or upon explicit application request.
When using the Opus codec, use of the built-in Opus FEC mechanism is RECOMMENDED. This provides reasonable protection of the audio stream against typical losses, with modest overhead. Note that as indicated above the built-in Opus FEC only provides single-frame redundancy; if multi-packet protection is needed, the built-in FEC should be combined with [RFC2198] redundancy to protect the N-2th, N-3rd, etc. packets.
When using the AMR/AMR-WB codecs, use of their built-in FEC mechanism is RECOMMENDED. This provides slightly more efficient protection of the audio stream than [RFC2198].
When using variable-bitrate codecs without an internal FEC, [RFC2198] redundant encoding with lower-fidelity version(s) of previous packet(s) is RECOMMENDED. This provides reasonable protection of the payload with moderate overhead.
When using constant-bitrate codecs, e.g. PCMU, use of [RFC2198] redundant encoding MAY be used, but note that this will result in a potentially significant bitrate increase, and that suddenly increasing bitrate to deal with losses from congestion may actually make things worse.
Because of the lower packet rate of audio encodings, usually a single packet per frame, use of a separate FEC stream comes with a higher overhead than other mechanisms, and therefore is NOT RECOMMENDED.
Support for redundant encoding MUST be indicated by offering "red" as a supported payload type in the offer. Answerers can reject the use of redundant encoding by not including "red" as a supported payload type in the answer.
Support for codec-specific FEC mechanisms are typically indicated via "a=fmtp" parameters.
For Opus, a receiver MUST indicate that it is prepared to use incoming FEC data with the "useinbandfec=1" parameter, as specified in [RFC7587]. This parameter is declarative and can be negotiated separately for either media direction.
For AMR/AMR-WB, support for redundant encoding, and the maximum supported depth, are controlled by the 'max-red' parameter, as specified in [RFC4867], Section 8.1. Receivers MUST include this parameter, and set it to an appropriate value, as specified in [3GPP.26.114], Table 6.3.
The following section provides guidance on how to best use FEC for transmitting video data. As indicated in Section 8 below, FEC should only be activated if network conditions warrant it, or upon explicit application request.
For video content, use of a separate FEC stream with the RTP payload format described in [I-D.ietf-payload-flexible-fec-scheme] is RECOMMENDED. The receiver can demultiplex the incoming FEC stream by SSRC and correlate it with the primary stream via the SSRC field present in the FEC header.
Support for protecting multiple primary streams with a single FEC stream is complicated by WebRTC's 1-m-line-per-stream policy, which does not allow for a m-line dedicated specifically to FEC.
To offer support for a SSRC-multiplexed FEC stream that is associated with a given primary stream, the offerer MUST offer the formats supported for the primary stream, as well as one of the formats described in [I-D.ietf-payload-flexible-fec-scheme], Section 5.1.
Use of FEC-only m-lines, and grouping using the SDP group mechanism as described in [RFC5956], Section 4.1 is not currently defined for WebRTC, and SHOULD NOT be offered.
Answerers can reject the use of SSRC-multiplexed FEC, by not including FEC formats in the answer.
Answerers SHOULD reject any FEC-only m-lines, unless they specifically know how to handle such a thing in a WebRTC context (perhaps defined by a future version of the WebRTC specifications). This ensures that implementations will not malfunction when said future version of WebRTC enables offers of FEC-only m-lines.
WebRTC also supports the ability to send generic application data, and provides transport-level retransmission mechanisms to support full and partial (e.g. timed) reliability. See [I-D.ietf-rtcweb-data-channel] for details.
Because the application can control exactly what data to send, it has the ability to monitor packet statistics and perform its own application-level FEC, if necessary.
As a result, this document makes no recommendations regarding FEC for the underlying data transport.
To support the functionality recommended above, implementations MUST be able to receive and make use of the relevant FEC formats for their supported audio codecs, and MUST indicate this support, as described in Section 4. Use of these formats when sending, as mentioned above, is RECOMMENDED.
The general FEC mechanism described in [I-D.ietf-payload-flexible-fec-scheme] SHOULD also be supported, as mentioned in Section 5.
Implementations MAY support additional FEC mechanisms if desired, e.g. [RFC5109].
Since use of FEC always causes redundant data to be transmitted, this will lead to less bandwidth available for the primary encoding when in a bandwidth-constrained environment. This is in contrast to methods like RTX [RFC4588], which only transmits redundant data when necessary, at the cost of an extra roundtrip.
Given this, WebRTC implementations SHOULD consider using RTX instead of FEC when RTT is low, and SHOULD only transmit the amount of FEC needed to protect against the observed packet loss (which can be determined, e.g., by monitoring transmit packet loss data from RTCP Receiver Reports [RFC3550]), unless the application indicates it is willing to pay a quality penalty to proactively avoid losses.
When using FEC with layered codecs, e.g., [RFC6386], where only base layer frames are critical to the decoding of future frames, implementations SHOULD only apply FEC to these base layer frames.
This document makes recommendations regarding the use of FEC. Generally, it should be noted that although applying redundancy is often useful in protecting a stream against packet loss, if the loss is caused by network congestion, the additional bandwidth used by the redundant data may actually make the situation worse, and can lead to significant degradation of the network.
Additional security considerations for each individual FEC mechanism are enumerated in their respective documents.
This document requires no actions from IANA.
Several people provided significant input into this document, including Bernard Aboba, Jonathan Lennox, Giri Mandyam, Varun Singh, Tim Terriberry, Magnus Westerlund, and Mo Zanaty.
| [I-D.ietf-payload-flexible-fec-scheme] | Singh, V., Begen, A., Zanaty, M. and G. Mandyam, "RTP Payload Format for Flexible Forward Error Correction (FEC)", Internet-Draft draft-ietf-payload-flexible-fec-scheme-03, October 2016. |
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
| [RFC2198] | Perkins, C., Kouvelas, I., Hodson, O., Hardman, V., Handley, M., Bolot, J., Vega-Garcia, A. and S. Fosse-Parisis, "RTP Payload for Redundant Audio Data", RFC 2198, DOI 10.17487/RFC2198, September 1997. |
| [RFC5956] | Begen, A., "Forward Error Correction Grouping Semantics in the Session Description Protocol", RFC 5956, DOI 10.17487/RFC5956, September 2010. |
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